Master the "two-seam" fastball. Once you effectively use the two-seam grip and arm motion, creating sinking action on the ball, you will be better prepared to throw sliders. For the two-seam fastball, place your index finger and your middle finger directly on the narrow seams of the ball -- the top of the "U" in the stitching. Put your thumb on the bottom of the ball, on the smooth surface of the ball directly under the fingers. Grip the ball tightly to create friction and movement. Use the same arm motion as on a regular or "four-seam" fastball.
Grip the outside of the ball along the long seam. Set your index and middle fingers along the inside of the right seam if you are right-handed, and along the inside of the left seam if you are left-handed. Place your thumb under the ball, opposite from your fingers, at a 45-degree angle. Make the ball roll off your index finger as you snap your wrist, creating a spin that takes the ball down and across the plate. Do not twist your elbow or wrist. "Most good slider pitchers grip the outer-third of the baseball and cock their wrist slightly, but not stiffly, to their throwing hand's thumb-side upon release of the pitch," pitching instructor Steven Ellis wrote on his website, The Complete Pitcher. "This enables a pitcher to apply pressure to the outer-half of the ball with the index finger."
Throw this pitch with the same motion and arm speed as your fastball. "The key to a good slider is making sure that it feels like a fastball coming off my fingertips and ends up looking like a fastball for 59 feet," IMG Academy pitching instructor Dave Shepard said in an IMG instructional video. "The last foot ends up looking like a slider."